1. Field of the Invention
This invention pertains to a process for covering edges and/or filling openings or gaps that are formed when photopolymerizable printing plates or photopolymer printing forms are installed on a printing cylinder for flexographic printing. In the process, edge covering material or gap filling material is applied and hardened by exposure to actinic radiation.
2. Description of Related Art
The preparation of flexographic printing forms from photopolymerizable printing plates is known. The printing surface is produced by imagewise exposure of a photopolymerizable layer by actinic radiation. Unexposed, unphotopolymerized areas of the printing plate are washed off. Examples of the preparation of flexographic printing forms are found in the following patents: DE-C 22 15 090, U.S. Pat. No. 4,266,005, U.S. Pat. No. 4,320,188, U.S. Pat. No. 4,126,466 and U.S. Pat. No. 4,430,417.
The printing form is prepared by exposing the photopolymerizable printing plate through a photographic original, washing off the unexposed areas with a solvent, and optionally, followed by a chemical post treatment and/or an overall postexposure. These processing steps can also be conducted optionally on a printing cylinder.
The photopolymer flexographic printing form, or if the printing form is produced on the cylinder, the photopolymerizable printing plate, is mounted on the printing cylinder or on an endless belt. In practice, the printing form is often assembled from individual pieces. Such assembly might be required, for example, by the subject being printed, for reasons of economy, or to achieve a larger plate format. In addition, for a range of end uses, it is necessary to cover the printing cylinder with the printing form in an endless manner to achieve a continuous printing surface. For this purpose, the printing plate or printing form is placed around the cylinder so that the ends abut and are adhered by double-sided adhesive tapes. If printing forms comprising individual pieces are used, the edges of the individual pieces are similarly adhered.
Gaps that interrupt the printing surface are formed between the abutting edges of the flexographic printing forms. Such gaps must be sealed in a suitable manner to prevent penetration by ink during printing, which would result in loosening the attachments of the printing forms and in undesirable impressions from the gaps. Sealing the gaps is also required for good synchronous operation of the printing cylinder, which an open gap does not deliver.
Gaps can also occur between the side edges of printing forms and the printing cylinder surface. These must be sealed against ink penetration.
Various gap fillers have already been proposed. DE-B 36 00 774 and DE-B 37 44 243 disclose photopolymerizable mixtures having special thermoplastic, elastomer, block copolymers as the main components. GB-B 2,160,882 discloses such mixtures having 100 parts of a diene-type prepolymer and 5-100 parts of an ethylenically unsaturated compound. These are applied in the gap and hardened by irradiation. Photopolymerizable mixtures having unsaturated polyesters (DE-A 39 20 093) and oxide-type fillers (DE-A 37 36 180) are also used as essential components to seal gaps occurring with gravure printing forms.
However, these gap fillers are often not stable enough to printing ink solvents and are not able to withstand the mechanical demands of the printing process. This causes cracks in the gap filler, which can now again fill with ink and thus result in undesirable print images. The applicability of known gap fillers is often poor and complicated.